The present invention relates to a screw pump having a pair of intermeshing screw rotors.
As a conventional screw pump, a displacement machine for compressible medium is disclosed by Japanese Patent Application Publication No. 2001-55992. The displacement machine includes two shafts and two intermeshing rotors which are fixed on the two shafts, respectively. The shafts are rotatably supported by bearings in a pump casing of the displacement machine. As the rotors are rotated, medium is drawn into a pump room of the displacement machine through an inlet port of the displacement machine and is discharged out of the displacement machine from the pump room through an outlet port of the displacement machine. Each shaft is provided with its own electric motor, and the rotor on the shaft is driven by the electric motor. Two intermeshing gears are provided at the bottom on the shafts.
FIG. 9 shows the rotors which are designated by reference numeral 80. Each rotor 80 has an inlet opening 82, a changing lead portion 85, and constant lead portions 83, 84. The inlet opening 82 is formed in the end face of the rotor 80 adjacent to an inlet port. Lead angle of the changing lead portion 85 of the rotor 80 decreases from the end face thereof toward the constant lead portion 84. Lead angles of the constant lead portions 83, 84 are constant. The rotors 80 and a housing of the displacement machine (not shown) define an inlet space P and a plurality of closed pump spaces S. The inlet space P is in communication with the inlet port through the inlet opening 82, so that fluid is drawn into the inlet space P during the rotation of the rotors 80. The closed pump spaces S are formed adjacent to the inlet space P. The inlet space P changes its volume while the rotors 80 make a complete one turn, and the inlet space P is transferred to a pump space S when the rotors 80 have completed the one turn.
In this case, when the rotors 80 have completed the one turn, the fluid in the inlet space P is transferred to the pump space S. Thus, the volume of fluid of the closed pump space S is the fluid volume to be transferred in the screw pump. If the lead angle of the rotor 80 is constant, the fluid volume of the inlet space P remains substantially constant without a change during the rotation of the rotors 80. That is, the fluid volume of the pump space S after rotation of the rotors 80 substantially coincides with that of the inlet space P before rotation of the rotors 80.
In the above conventional art, however, the volume of fluid of the closed pump space substantially is the volume to be transferred. The inlet space which is formed by the first one turn of the lead and in communication with the inlet port does not provide fluid compression. Merely setting the volume of the inlet space larger than that of the pump space will not improve the efficiency of drawing in the fluid into the inlet space. In addition, the conventional art wherein the volume of the inlet space is not effectively used, the rotor need to be lengthened in order to improve the efficiency of drawing in the fluid into the inlet space.
The present invention is directed to a screw pump wherein the inlet space which is provided by the first one turn of the lead is utilized for fluid transferring thereby to increase the volume of fluid to be transferred in the screw pump.